1. Field of the Invention
This invention relates to a toner used in image forming processes such as electrophotography, electrostatic printing, toner jet recording and magnetic recording, and an image forming method making use of the toner.
2. Related Background Art
With the spread of electrophotographic full-color image forming apparatus, a great variety of their uses have been developed, and a demand for image quality have now come severer. In the copying or printing of images such as catalogues and maps, the details thereof are required to be reproduced finely and faithfully. With such demands, a demand for the sharpness of colors also has been heightened, and it is desired to extend the range of color reproduction. In particular, in these days the advance of electrophotographic apparatus into the field of printing is marked, and also in electrophotographic systems, it has been required that colofulness, minuteness and graininess of image characteristics are improved to be at a level not lower than those in printing.
In recent years, in full-color image forming apparatus such as full-color copying machines having been proposed, commonly used are, e.g., a method in which, using a plurality of photosensitive members, electrostatic latent images formed respectively on the photosensitive members are developed with a cyan toner, a magenta toner, a yellow toner and a black toner to form corresponding toner images and then a transfer material is transported between the photosensitive members and a belt-like transfer member to transfer the toner images thereto in a straight pass, followed by fixing to form a full-color image, and a method in which the transfer material is wound around the surface of a cylindrical transfer member set opposite to a photosensitive member, by the aid of electrostatic force or by mechanical action of a gripper or the like, and the steps of development and transfer are carried out four times to obtain a full-color image.
Toners used in these full-color image forming apparatus such as full-color copying machines are required to be sufficiently color-mixed and be heat-fixed to a transfer material in the step of heat-and-pressure fixing, without damaging color reproducibility and transparency of overhead projector (OHP) images.
To meet such requirements, as binder resins used in toner particles, it is preferable to use resins having higher sharp-melt properties. In recent years, polyester resins are used as the resins having sharp-melt properties. As polymerization catalysts used for the production of polyester resins used in the binder resins of toner particles, tin type catalysts such as dibutyltin oxide and antimony type catalysts such as antimony trioxide have been in common use. These techniques are still not fully satisfactory because fixing performance such as low-temperature fixing performance or high-temperature anti-offset properties and color reproducibility such as color mixing performance or transparency are important as performances of toners in order to satisfy functions such as high speed, high image quality and high minuteness which are demanded in the full-color image forming apparatus such as copying machines for full-color images.
Accordingly, in Japanese Patent Applications Laid-open No. 2002-148867 and No. 2001-64378, techniques are disclosed in which titanates of aromatic diols are used as polymerization catalysts or solid titanium compounds are used as polymerization catalysts.
These proposals still have problems on fixing performance, color reproducibility and developing performance in some cases, and further improvement is necessary.
The use of the resins having sharp-melt properties usually tends to bring about a problem on high-temperature anti-offset properties because binder resins have a low self cohesive force when toners melt in the step of heat-and-pressure fixing. Accordingly, in order to improve the high-temperature anti-offset properties at the time of fixing, relatively highly crystalline waxes as typified by polyethylene wax and polypropylene wax are used as release agents.
However, especially in toners used in full-color images, because of such a high crystallizability of the release agents themselves and a difference in refractive index between toners and OHP sheet materials, the transparency may be damaged when projected through OHPs, resulting in a low chroma or brightness of projected images.
Accordingly, in order to solve such problems, as disclosed in Japanese Patent Applications Laid-open No. H4-149559 and No. H4-107467, a method is proposed in which a nucleating agent and a wax are used in combination to lower the crystallizability of the wax. As also disclosed in Japanese Patent Applications Laid-open No. H4-301858 and No. H5-61238, a method is further proposed in which a wax having low crystallinity is used. As other waxes, as disclosed in Japanese Patent Applications Laid-open No. H1-185660 and No. H1-238672, it is proposed to use montan type waxes as waxes having relatively good transparency and low melting points. These waxes, however, are not those which can satisfy all the transparency on OHPs, the low-temperature fixing performance at the time of heat-and-pressure fixing and the high-temperature anti-offset properties.